Optical resonator is a research highlight of laser optics, optical sensing and etc. One of the key parameters of optical resonator is finesse or quality factor, which generally determines the ultimate performance of the system. As for the resonator optical gyro, the finesse of the passive ring resonator determines the fundamental limit of the system, and meanwhile the nonreciprocal error of a transmissive-type system is lower than that of a reflective-type system; therefore, a transmissive optical ring resonator with high-finesse is important for resonator optic gyro to reach high precision.
Photonic crystal fiber (PCF) has many excellent properties but, it is completely different from the traditional optical fibers, so do the fiber optic components such as fiber ring resonator. A variety of PCF ring resonator structures have been proposed. Developing a high-performance PCF ring resonator and applying it to the resonator optic gyro becomes a research highlight of inertial sensing in recent years.
The principle of ring resonator is multi-beam interference. For a transmissive ring resonator, the constructive interference among the light beams circulated in the resonator forms a series of interference peaks at the output port, so its transfer function can be regarded as discrete bright fringes in dark background. For a reflective ring resonator, the destructive interference between the incident light beam and the sum of the light beams circulated in the resonator forms a series of interference dips at the output port, so its transfer function can be regarded as discrete dark fringes in bright background.
Limited by the principle of wave guiding in photonic crystal fiber, the PCF ring resonator is generally assembled with individual components at present. A conventional transmissive PCF ring resonator consists of two optical beam-splitters, four fiber-optic collimators and four PCF collimators—so many components would lead to an increase of round-trip loss of resonator and difficulty of regulation, which was not good for increasing the finesse of resonator.